Modem disk drive recording systems typically rely upon a flat, circular platter having a surface which has been coated with a magnetic material for the storage of information by selective polarization portions of the magnetic surface. In such systems, a transducer or head is positioned over the magnetic surface of the rotating disk. By energizing coils in the transducer, magnetic flux can be generated to polarized selective portions of the disk surface. Likewise, the magnetic flux emanating from the disk surface from previously recorded data can be detected by the transducer in a similar manner. The presence or absence of polarity transitions is recognized by the disk drive electronics by means of a set of binary signals. An electronic data processing circuit, commonly known as a read/write data channel, is employed for this purpose.
For example, during the reading process the read channel converts the voltage waveform of the transducer output signal to a digital waveform which should be the same as the original modified frequency modulation signal. A general discussion of data channel operation in a rigid disk drive assembly is provided in Magnetic Recording, Vol. 11 Computer Data Storage by Mee and Daniel, McGraw-Hill, 1988, pp. 155-162.
Experimental techniques for analyzing the performance of a digital magnetic recording read/write channel are well-known in the field of disk drives. Numerous factors can affect the strength of the polarization of the magnetic surface in a manner which interferes with normal data reproduction. For example, defects in the magnetic disks, crosstalk between adjacent tracks of the disk, and intersymbol interference in re-occurring noise transients all contribute to the data read/write error rate of the disk drive. Theoretical models have been presented to predict the most effective utilization of a read/write channel in order to improve the channel error rate in the face of the combined effects of the above listed factors. By way of background, an article entitled "Effective Bit Shift Distribution On Error Rate In Magnetic Recording," by Katz and Campbell, IEEE Transactions On Magnetics, Vol. MAG-15, No. 3, May, 1979, describes such a theoretical model.
For some time, practitioners have struggled to develop various techniques for eliminating the presence of false or missing pulses from the raw data signal. The goal of such techniques is generally to optimize the raw soft error rate performance of the disk drive channel. The raw soft error rate refers to the frequency of errors that occurs due to the susceptibility of the read channel signal to various factors such as crosstalk and intersymbol interference. In basic terms, the raw soft error rate means that the data reproduced should be the same as the data that were originally written to the magnetic medium.
To improve the error rate of a magnetic recording system, prior art approaches have generally provided for read/write data channels that are programmably adaptable in processing data signals transferred to and from the read/write head. For example, U.S. Pat. No. 5,121,262 describes programming elements for determining whether data signals transferred from the media exceed a programmable data discrimination level, and whether data signals transfer from the medium occur within a data signal window whose timing is relative to a data clock signal. Another example of an adaptive variable threshold qualification level circuit for disk drives is described in U.S. Pat. No. 5,150,050. In the latter system, a qualification level generator circuit generates a qualification level signal which varies in value depending upon the address portion of the magnetic disk.
One of the problems associated with prior art adaptive read/write channel controls has been their focus on the measurement criteria of timing margin and threshold margin. Additionally, past approaches rest on the assumption that disk drives always read on track; that is, that the head or transducer is optimally positioned over one of the concentric data tracks defined on the surface of the recording medium. In a disk drive system subject to real-world environmental influences, however, external forces such as mechanical shock and vibration often cause the head to move off-track.
What all of this means is that the true test of whether an adaptive algorithm works in an actual disk drive is not only related to how good its timing and threshold margins are on-track, but also how wide its erase bands are off-track. Once the transducers or heads are positioned slightly off-track, noise from adjacent tracks becomes a serious problem. Thus, a disk drive system may have its read/write channel performance optimized under ideal on-track conditions, while under real operating conditions the drive will either fail or perform non-optimally due to poor off-track margin control.
Therefore, what is needed is an adaptive read/write channel which can optimize the raw soft error rate performance of the disk drive, while taking into account the realistic environmental and operating conditions that the drive normally experiences.